A liquid crystal display (LCD) does not have self-luminescent properties and thus needs an additional light-emitting source. Such a light emitting source is disposed at the rear surface of a liquid crystal panel to cause light to be transmitted by the liquid crystal panel and be emitted, and thus the light emitting source is commonly called a back light unit (BLU).
A back light unit that functions as a planar light source is ideal. However, it is technically difficult to implement a planar light source having a uniform luminance over the entire surface. Therefore, a method of diffusing light emitted by a light source similar to a linear light source or a point light source such as a cold-cathode fluorescent lamp (CCFL) or a light-emitting diode (LED) to be adjusted to a state close to the planar light source is used. Here, a light guide plate (LGP) has an important role in diffusing the light.
The light guide plate is a plate material typically made of an acrylic resin and has a predetermined pattern formed so that light emitted by a light source disposed at the side surface or the bottom surface is uniformly diffused to the front surface. The pattern achieves a desired object by reflecting or refracting the light. According to the form of the pattern, luminance uniformity of the entire surface of the light guide plate is determined. Therefore, more complex and precise patterns are required of the light guide plate, and a reduction in time taken to form the patterns has emerged as an urgent issue with an increase in the area of a display apparatus.
In order to solve the problems, techniques of forming a pattern on an acrylic plate material using a laser beam have been developed.
FIG. 8 is a schematic plan view illustrating an example of a laser processing apparatus for a light guide plate, which forms a pattern on the light guide plate using a laser according to the related art, and illustrates a diagram of Korean Patent No. 460790.
The laser processing apparatus for light guide plate according to the related art includes: a Y-axis guide rail 62 which is relatively fixed; an X-axis guide rail 61 which moves along the Y-axis guide rail; a first mirror 58 fixed to the X-axis guide rail; and a second mirror 59 which moves along the X-axis guide rail. In this configuration, when a laser system 53 emits a laser beam toward the first mirror 58, the laser beam is reflected by the first mirror 58 toward the second mirror 59, and the laser beam that is reflected again by the second mirror 59 reaches a light guide plate 41 positioned below the second mirror 59, thereby forming a predetermined light guide pattern portion 45 on the light guide plate 41.
In order to form the light guide pattern portion 45 over the entire surface of the light guide plate 41, the X-axis guide rail 61 and the first mirror 58 fixed thereto have to move along the Y-axis guide rail 62, and the second mirror 59 has to move along the X-axis guide rail 61. On the other hand, since the laser system 53 and the light guide plate 41 maintain the relatively fixed positions, the light path of the laser beam from the laser system 53 and the light guide plate 41 is frequently changed.
When the light path of the laser beam is changed as described above, the cross-sectional diameter of the laser beam is changed when the laser beam reaches the light guide plate 41. For example, when the light path is lengthened, the laser beam is further diffused, and thus the cross-sectional diameter thereof is also increased. A change in the cross-sectional diameter of the laser beam changes the size of the light guide pattern portion 45, and also changes the depth of light guide pattern portion 45 due to a reduction in energy density. As a result, a pattern having sizes and depths different from those of the pattern to be formed is formed on the light guide plate. Therefore, there is a problem in that the quality uniformity of a completely processed light guide plate is degraded.